Channel-length impact on radiation-induced threshold-voltage shift in N-MOSFET's devices at low gamma rays radiation doses

SiO2 gate dielectric and Si/SiO2 interface are two important components, wh ich will shape the future of the MOSFETs and integrated circuits (IC's) tec hnologies for ionizing radiation environment applications. This study discu sses their size effect on irradiated NMOS device response. N-channel MOSFET 's of different gate sizes were first irradiated with Co-60 Gamma rays sour ce at several total doses (low doses). Then, they were characterized by usi ng both voltage- and frequency-charge pumping (CP) techniques. On one hand, all transistors reveal two radiation-induced oxide-trap format ion mechanisms, caused by low radiation total doses. Initially, there is a positive charge build-up in the oxide layer, followed later by diminution o f net positive charge (turn around effect). The interface traps exhibit a l inear increase with radiation dose. On the other hand, the response of irradiated transistors is shown to depen d on their gate lengths. However, transistors with shorter gate lengths sho w more negative threshold-voltage shifts (DeltaV(th)) during irradiation th an transistors with longer gate lengths. We have found that this behavior i s caused by oxide traps. They induce a real difference in DeltaV(ot) shift in transistors of different channel length. The DeltaV(it) shift remains re latively unchanged.